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Full text of "Text Book Of Mechanical Engineering"

892

Appendix II.

P. 618. The Indicator.—Certain errors are possible in all
indicators and their gears, which should be eliminated or allowed
for. (i) The pressure springs should be tested by connecting the
indicator to a mercury gauge, and admitting steam to both
simultaneously, up to full load. A coefficient is thus deduced to
correct indicator readings. (2) The drum spring being light, is
sufficiently corrected by the use of wires instead of strings. (3)
The decreasing gear should be mechanically accurate and rigid.
(4) The connecting pipes PR Q, Fig. 621, are best dispensed with
for very accurate purposes, and two indicators used, one at p and
the other at Q.

P. 627.    Willans' Law was discovered by the late P. W.

TJuz

Willans when testing his engines. It occurred to him to plot
total steam consumption per hour to a base of H.P. (brake,
indicated, or electrical) and as the latter was varied by throttling
the steam at constant expansion, he found the plottings to follow
an inclined straight line passing above the origin, shewing total
steam consumption to be proportional to H.P. plus a constant. In
Fig- 853 is a pair of these lines for Willans engines, where the
heights o A and o B represent steam consumption for no work, viz.,
condensation and leakage. Gas-engine lines pass nearly through
o, but oil engines have similar losses to steam engines. Another
method of plotting is to use piston mean-pressure as base, Fig* 85 4,
and two lines are shewn for different expansion ratios in a good
condensing engine, the steam consumption being reckoned
^oreticaHy as on p. 627, but neglecting factors Df and Lq ; there
herefore, no loss at the origin. Similarly the dotted line is the